The Mysterious Two Pop Explained

Aaron Owen

Jun 4
4 min read

Cinematiq’s Source Material Specs specify that all films submitted for Digital Cinema Package (DCP) authoring need to have visual and audible two and tail pops. If you have no idea what this means, you’re in the right place. I’ll explain what they are, how to create them, and why they are important.

You’ve probably seen some version of the SMPTE Universal Leader. It’s a pretty famous piece of film and often serves as a shortcut to nostalgia for folks of a certain age. It’s a series of numbers counting backwards from 9 to 3 with a single frame of the number two at the end. It turns out that there’s a real technical reason that this countdown was invented, and why even in this digital day and age, it still matters somewhat.

More Than a Retro Graphic

One of the main purposes behind the leader was to provide a strip of film to help thread the projector. It also provides a reference frame to sync sound to. Because we’re in the digital age of film, there’s no need to have a leader to help the projector’s motor get up to speed, but we do still need a way to sync the sound with the picture. If you take a look at the leader frame by frame, there’s 24 frames for each number in the countdown with the exception of the number two, which is represented with a single frame.

A frame by frame look at the universal leader.

This is because film projectors run at a frame rate of 24 frames per second. It was true in the 50’s and 60’s and it is still true today. By having the first frame be a different color it was easy for the projectionist to see where each number started by looking at the film against the light. If you watch the countdown in real-time, you’d have a tough time spotting the very last frame since it is on screen for only 1/24th of a second, but it’s there. That frame, which is only one frame of the number 2, is called the “two-pop” and it is the very last frame of the countdown.

After the two-pop, there is exactly two seconds of black before the first frame of the actual film. This is so that the projectionist could “park” the projector in black and still have two seconds for the projector to get up to speed before the picture started playing on screen.

Two-Pop for Sync Sound

The two-pop gets it’s name from the combination of the number two on the screen, and a single frame of sound of 1kHz tone on the audio track. This “pop” of sound showed technicians where to align the soundtrack with the picture so that the audio would be in sync. The practice of using a two-pop to sync sound is still in use by professional audio studios doing sound for film or television. In practice, whenever there are multiple channels (stereo or 5.1 surround sound), a pop must be placed on each channel to ensure that every channel stays in sync not only with the picture, but with the other audio channels.

When Cinematiq authors your DCP, the two-pop is how we make sure that your multi-channel audio stays in sync. We find the pop on each of your audio channels, and line it up with the two-pop frame. If we don’t have a pop on each channel of audio, it isn’t possible to know if that particular channel is in sync for sure.

The tail pop is usually turned upside down.

The Two-Pop’s Cousin the Tail-Pop

If the film starts in sync, how do you ensure that over the course of the duration of the film the audio doesn’t slip out of sync? If audio and picture are in sync at the two-pop frame, and sync drifts or somehow loses sync, it won’t be possible to tell how far out of sync the audio is if there’s not a sync marker at the end of the film as well. This is what’s known as a tail-pop. The tail-pop is placed exactly two seconds after the film’s last frame (usually either a fade or cut to black from the credits or a copyright notice) and also gets exactly one frame of 1kHz tone.

There’s a long tradition in the movie business that any marker that refers to what’s come before it gets turned upside down. Just like tail-slates are turned upside-down on set, the tail-pop also gets turned upside-down to indicate that it is a tail-pop. This little detail is mostly irrelevant in today’s digital world where we’re usually looking at films in a non-linear timeline rather than from a linear filmstrip or videotape where it would be much more difficult to know what the marker was supposed to mean. Turning your tail-pop upside down usually is an indicator that you know what you’re doing.

What’s That Frame Rate Again?

The tail-pop is really important in DCP authoring because of the difference between 23.976 video and 24fps film. As mentioned above, movie theater projectors still project movies at true 24 frames per second (fps). Video on the other hand, runs at a really strange frame rate because of archaic legacy issues in the television world. Most video cameras 24p modes actually run at 23.976 which is sometimes displayed as 23.98. If you’ve shot your film on a “video” (or digital cinema) camera, chances are your frame rate is actually 23.976.

Computers don’t really care about the strange frame rate because their screens refresh at a much higher rate. Video monitors and televisions usually support them out of the box and sometimes mis-label 23.976 as “24p”. The only place where this difference really matters is when you want to exhibit your film, shot using video frame-rates, in a traditional movie theater using digital cinema gear. It is in this situation that your film needs to be converted to a true 24fps. The experts at Cinematiq handle this conversion for you, but we need the tail-pop to ensure that at the end of the process, your film is 100% in sync. Without the tail-pop, we’re flying blind, and can’t guarantee that sync is frame accurate.

How to Make a Two/Tail Pop

Making a two-pop or a tail-pop is fairly simple, yet it gets screwed up a lot. In order to not have your source materials rejected (and overages applied), please take a look at this tutorial to see how you can ensure that your film has a proper two and tail pop.

Now that your film is ready for DCP authoring, check out this article on delivering your DCP to theaters.

About Cinematiq: We’re a digital motion picture lab based in San Francisco specializing in video encoding and DCP creation. Our team of technicians and video specialists work to make sure that your vision ends up on screen the way it was intended. Whether you are a filmmaker, festival or distributor, we’ve got solutions to some of the most challenging problems in the motion picture industry today. We’re also the creators of DCP Transfer, a software application that allows you to easily format hard drives using the ext2 filesystem for DCP Delivery, as well as view DCP info / metadata.

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